A nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode material and preparation method thereof

Abstract

This invention discloses a networked nitrogen-containing conjugated lignin cathode material and its preparation method. By utilizing the trifunctionality of melamine, it is gradually polymerized with brominated lignin to form a nitrogen-containing conjugated lignin polymer with a hierarchical porous structure. The nitrogen-containing conjugated lignin cathode material provided by the invention has a simple preparation process and low cost. It exhibits a high initial specific capacity and low internal resistance for lithium-sulfur batteries, significantly improving the performance of lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of positive electrode materials for lithium-sulfur batteries, and relates to a nitrogen-containing porous structure lignin conjugated polymer lithium-sulfur battery positive electrode material for lithium-sulfur batteries and a preparation method thereof. Background Art

[0002] Lithium-sulfur (Li-S) batteries are considered to be of great significance for new energy storage technologies such as portable electronics and electric vehicles due to their high theoretical capacity (1675mAh / g) and energy density (~2500Wh / kg). However, during the charge and discharge process, the dielectric properties and volume expansion of sulfur-containing cathode materials significantly reduce the specific capacity and cycle life of the battery. To overcome these problems, people have overcome the problems of poor conductivity and volume change by improving the conductivity, specific surface area, and surface polarity of sulfur-containing cathodes. Therefore, carbon-based materials with porous structures and large specific surface areas, such as porous carbon, carbon nanotubes, carbon fibers, graphene, and reduced graphene oxide, are often used as conductive agents or polysulfide adsorption materials.

[0003] Although polysulfides can be dispersed in various porous materials, the "shuttle effect" of polysulfides cannot be suppressed during charge and discharge. Currently, MXenes can form Lewis acid-base interactions with polysulfides, but in practical applications, MXenes have the problem of irregular stacking.

[0004] Therefore, the development of inexpensive organic conjugated microporous polymers and the construction of conjugated polymers with microporous structures by covalently linking rigid, π-conjugated structural units with three or more functions are conducive to the effective capture of polysulfides in the sulfur cathode, thereby inhibiting polysulfide shuttling, which is very beneficial for electron / ion transport. Summary of the Invention

[0005] In light of this, the present invention provides a networked nitrogen-containing conjugated lignin cathode material and a lithium-sulfur battery. The networked nitrogen-containing conjugated lignin, based on lignin, is inexpensive and its network structure effectively shortens the lithium ion transport path. Furthermore, the cathode material's nitrogen-containing functional groups act as active sites in the conjugated polymer backbone, inhibiting polysulfide shuttling and further improving the electrochemical performance of the lithium-sulfur battery.

[0006] The present invention also provides a method for preparing the networked nitrogen-containing conjugated lignin positive electrode material. The raw materials for the preparation are readily available, the cost is low, and the material is environmentally friendly.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A method for preparing a nitrogen-containing conjugated lignin composite material, the method specifically comprising the following steps:

[0009] (1) taking desugared lignin and anhydrous dichloromethane, stirring, adding boron tribromide to react under nitrogen protection in an ice-water bath, then adding ice water to quench the catalyst, centrifuging, and removing the solvent by rotary evaporation to obtain brominated lignin;

[0010] (2) under nitrogen protection, dissolving the brominated lignin obtained in step (1), melamine, and a catalyst in anhydrous toluene, stirring, and heating under reflux, cooling to room temperature after the reaction is completed, centrifuging, washing, and drying to obtain a networked nitrogen-containing conjugated lignin polymer;

[0011] (3) Under an inert protective atmosphere, the networked nitrogen-containing conjugated lignin polymer obtained in step (2) is sintered with sulfur powder at a high temperature to obtain the nitrogen-containing conjugated lignin composite material.

[0012] Optionally, the lignin is one or a mixture of two or more of alkali lignin, enzymatic lignin, sulfate lignin, sulfonate lignin or other modified lignins.

[0013] Optionally, in step (1), the mass volume ratio of the lignin to anhydrous dichloromethane is (1-4) g: (100-400) mL, and the mass ratio of the lignin to boron tribromide is (0.5-1.5): (1-10).

[0014] Furthermore, the reaction temperature is 0-4°C, and the reaction time is 1-6 hours.

[0015] Optionally, in step (2), the mass ratio of the brominated lignin to melamine is (0.5-1.5):(1-2), and the mass volume ratio of the brominated lignin to toluene is (1-2) g:(200-600) mL;

[0016] The catalyst is at least one of bis(dibenzylideneacetonepalladium), 2-dicyclohexylphosphine-2',4',6'-triisopropylbiphenyl, and sodium tert-butoxide, and the mass ratio of the brominated lignin to bis(dibenzylideneacetonepalladium), 2-dicyclohexylphosphine-2',4',6'-triisopropylbiphenyl, and sodium tert-butoxide is 100:(1-10):(1-10):(1-10).

[0017] Furthermore, the heating reflux temperature is 110°C and the reflux time is 6 to 48 hours. The electrochemical performance of the material prepared by selecting this range is optimal. N,N-dimethylformamide, chloroform and glacial acetic acid are used for washing 3 times each to obtain a networked nitrogen-containing conjugated lignin polymer, and the mass ratio of the nitrogen-containing conjugated lignin polymer to N,N-dimethylformamide, chloroform and glacial acetic acid is (0.5 to 1.5):(20 to 100):(20 to 100):(1 to 20).

[0018] Optionally, the mass ratio of the nitrogen-containing conjugated lignin polymer to sulfur powder is 1:(1-10), and the high-temperature sintering process is:

[0019] Under an inert protective atmosphere, the nitrogen-containing conjugated lignin polymer and sulfur powder are heated at 150°C to 180°C for 12 hours, and then kept at 180°C to 230°C for 0.5 to 2 hours. The material prepared in this range has the best electrochemical performance.

[0020] The present invention also claims protection for a nitrogen-containing conjugated lignin composite material prepared by the above method, wherein the material has a lignin conjugated polymer network structure, and the nitrogen-containing functional groups in the polymer can serve as active sites to inhibit "polysulfide shuttling".

[0021] Furthermore, the present invention seeks to protect the use of the nitrogen-containing conjugated lignin composite material prepared by the above method in lithium-sulfur batteries.

[0022] Specifically, the obtained nitrogen-containing conjugated lignin composite material is ground and mixed with SuperP and polyvinylidene fluoride PVDF to obtain a positive electrode slurry; the obtained positive electrode slurry is then spread on the surface of aluminum foil and vacuum-dried to obtain a nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode; wherein,

[0023] The mass ratio of the nitrogen-containing conjugated lignin composite material to SuperP and polyvinylidene fluoride (PVDF) is (6-9.5):(0.5-2):(0.5-2).

[0024] A scraper with a height of 6 to 20 μm is used to scrape the nitrogen-containing conjugated lignin positive electrode slurry; the vacuum drying temperature is 50° C. to 80° C., and the vacuum drying time is 6 to 24 hours. The lithium-sulfur battery positive electrode prepared within this range is not prone to cracking or falling off.

[0025] It can be seen from the above technical solution that, compared with the prior art, the nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode material and the preparation method thereof provided by the present invention have the following excellent effects:

[0026] 1. The nitrogen-containing conjugated lignin prepared by the present invention is a porous polymer with simple production process and low price that can be used as a positive electrode material for lithium-sulfur batteries.

[0027] 2. The nitrogen-containing conjugated lignin prepared by the present invention has rich pores and a specific surface area of 274.6 m 2 / g; average pore size of 8.3nm; well-developed mesopores, conducive to achieving high sulfur content. The porous structure not only improves sulfur utilization, but also serves to confine polysulfides within the pores and inhibit their free migration to the anode. Nitrogen-containing conjugated lignin, as a lithium-sulfur battery cathode material, has an initial discharge capacity of 906mAh / g at 0.1C. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0029] Figure 1 Cyclic voltammetry curve of lithium-sulfur battery containing nitrogen-conjugated lignin at a scan rate of 0.1 mV / s.

[0030] Figure 2 Rate performance of lithium-sulfur batteries based on nitrogen-conjugated lignin.

[0031] Figure 3 The cycle life curve of lithium-sulfur battery containing nitrogen-conjugated lignin.

[0032] Figure 4 This is the EIS impedance spectrum of lithium-sulfur battery containing nitrogen-conjugated lignin.

[0033] Figure 5 is the pore volume and pore size distribution curve of nitrogen-containing conjugated lignin. DETAILED DESCRIPTION

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] For a better understanding of the present invention, the present invention is further specifically described below through the following examples, but it should not be understood as limiting the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above invention content are also considered to fall within the scope of protection of the present invention.

[0036] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0037] Example 1:

[0038] Under a nitrogen atmosphere, 4 mL of boron tribromide was added to a suspension mixture of 1 g of alkali lignin and anhydrous dichloromethane. After reacting at 4°C for 4 hours, the boron tribromide was quenched, and the reaction product was centrifuged. 190 mg of melamine, 35 mg of bisdibenzylideneacetone palladium, 43 mg of 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl and 385 mg of sodium tert-butoxide were added, and 50 mL of anhydrous toluene was added. The mixture was stirred evenly and refluxed for 24 hours. The mixture was cooled to room temperature, centrifuged, washed three times with 50 mL of N,N-dimethylformamide, chloroform and glacial acetic acid respectively, and freeze-dried to obtain a nitrogen-containing alkali lignin polymer.

[0039] The nitrogen-containing alkali lignin polymer and sublimed sulfur were mixed and ground in a mass ratio of 1:3, and heated in an argon environment at 155°C for 12 hours to obtain a nitrogen-containing conjugated lignin composite material.

[0040] The composite material was mixed with SuperP and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1 and ground to obtain a positive electrode slurry. The positive electrode slurry was poured onto a clean aluminum foil surface and scraped with a 10μm-high scraper. The film was then treated at 50°C under vacuum for 12 hours to obtain the nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode material, which was then cut into electrodes with a diameter of 14mm.

[0041] It was measured that the mass of the prepared nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode active material was 1.38 mg, and the button battery was prepared using 80 μL of electrolyte, and its initial specific capacity was 906.21 mAh / g.

[0042] Example 2:

[0043] The positive electrode slurry in Example 1 was used, and a scraper with a height of 10 μm was used to scrape the film. The nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode prepared by the method in Example 1 had an active material mass of 2.1 mg. 80 μL of electrolyte was used to prepare a button battery, and its initial specific capacity was 576.24 mAh / g.

[0044] Example 3:

[0045] The positive electrode slurry in Example 1 was used, and a scraper with a height of 15 μm was used to scrape the film. The nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode was prepared by the method in Example 1. The active material mass was 1.56 mg, and 80 μL of electrolyte was used to prepare a button battery. Its initial specific capacity was 809.31 mAh / g.

[0046] Example 4:

[0047] The positive electrode slurry in Example 1 was used, and a scraper with a height of 20 μm was used to scrape the film. The nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode was prepared by the method in Example 1. The active material mass was 1.92 mg, and 80 μL of electrolyte was used to prepare a button battery. Its initial specific capacity was 520.79 mAh / g.

[0048] Example 5:

[0049] The positive electrode slurry in Example 1 was used, and the film was scraped with a scraper with a height of 20 μm. After laying the film according to the method in Example 1, the film was dried for 8 hours to prepare a nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode. The active material mass was 1.92 mg, and 60 μL of electrolyte was used to prepare a button battery. Its initial specific capacity was 669.75 mAh / g.

[0050] In addition, in order to further verify the superiority of the technical solution of the present invention compared with the existing technology, the inventors also conducted experimental measurements, and the specific experimental results are analyzed as follows:

[0051] The cyclic voltammetry curve of the nitrogen-containing conjugated lignin lithium-sulfur battery prepared in Example 1 shows the characteristic redox characteristics of Li-S batteries, with two obvious reduction peaks appearing at about 2.05V and 2.35V (see Appendix Figure 1 ), which can be attributed to the transformation from S8 molecules to higher-order LiPS and further reduction to Li2S2 and Li2S. At 0.1C, the initial discharge capacity of the battery was 906.21mAh / g, reaching 54.1% of the theoretical capacity (1675mAh / g, see Appendix Figure 2 ), which is much higher than the specific capacitance of general conjugated organic porous polymers.

[0052] At 0.1C, the battery has a specific capacity of 278.64mAh / g after 50 cycles of constant current charge and discharge (see attached Figure 3 ), indicating that the material decays rapidly during high current charge and discharge.

[0053] The impedance curve of the battery consists of a quasi-semicircle in the high-frequency region and a straight line in the low-frequency region. By fitting the EIS curve using a commonly used equivalent circuit, it was found that the solution resistance Rs of the lithium-sulfur battery prepared with nitrogen-containing conjugated lignin was 5.3116Ω, with a lower charge transfer 电阻 Rct(72.06Ω)(see attached Figure 4 Before the cycle test, the battery was first activated at 0.1C for three cycles. Figure 4 It shows that the battery prepared by S@CMP-M has an initial specific capacity of 906.21 mAh / g at 0.1C, which has a high initial specific capacity.

[0054] Therefore, based on the above analysis, it can be seen that the lithium-sulfur battery prepared from nitrogen-containing conjugated lignin with a porous structure has a high initial specific capacity, and the present invention provides an application method for lignin-modified conjugated polymers as inexpensive lithium-sulfur battery positive electrode materials.

[0055] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a nitrogen-containing conjugated lignin composite material, characterized in that: The method specifically comprises the following steps: (1) taking desugared lignin and anhydrous dichloromethane, stirring, adding boron tribromide to react under nitrogen protection in an ice-water bath, then adding ice water to quench the boron tribromide, centrifuging, and removing the solvent by rotary evaporation to obtain brominated lignin; (2) under nitrogen protection, dissolving the brominated lignin obtained in step (1), melamine, and a catalyst in anhydrous toluene, stirring, and heating under reflux, cooling to room temperature after the reaction is completed, centrifuging, washing, and drying to obtain a networked nitrogen-containing conjugated lignin polymer; (3) sintering the networked nitrogen-containing conjugated lignin polymer obtained in step (2) with sulfur powder at high temperature under an inert protective atmosphere to obtain the nitrogen-containing conjugated lignin composite material; Among them, the high temperature sintering process is: Under an inert protective atmosphere, the nitrogen-containing conjugated lignin polymer and sulfur powder are heated at 150°C-180°C for 12 hours, and then kept at 180°C-230°C for 0.5-2 hours.

2. The method for preparing the nitrogen-containing conjugated lignin composite material according to claim 1, wherein In step (1), the mass volume ratio of the lignin to anhydrous dichloromethane is (1-4) g:(100-400) mL, and the mass ratio of the lignin to boron tribromide is (0.5-1.5):(1-10).

3. The method for preparing the nitrogen-containing conjugated lignin composite material according to claim 1 or 2, characterized in that: In step (1), the reaction temperature is 0-4°C and the reaction time is 1-6h.

4. The method for preparing the nitrogen-containing conjugated lignin composite material according to claim 1, wherein In step (2), the mass ratio of the brominated lignin to melamine is (0.5-1.5):(1-2), and the mass volume ratio of the brominated lignin to toluene is (1-2) g:(200-600) mL; The catalyst is at least one of bis(dibenzylideneacetonepalladium), 2-dicyclohexylphosphine-2',4',6'-triisopropylbiphenyl, and sodium tert-butoxide, and the mass ratio of the brominated lignin to bis(dibenzylideneacetonepalladium), 2-dicyclohexylphosphine-2',4',6'-triisopropylbiphenyl, and sodium tert-butoxide is 100:(1-10):(1-10):(1-10).

5. The method for preparing the nitrogen-containing conjugated lignin composite material according to claim 1 or 4, characterized in that: The heating reflux temperature is 110° C., and the reflux time is 6 to 48 hours. The networked nitrogen-containing conjugated lignin polymer is obtained by washing with N,N-dimethylformamide, chloroform, and glacial acetic acid three times each, and the mass ratio of the nitrogen-containing conjugated lignin polymer to N,N-dimethylformamide, chloroform, and glacial acetic acid is (0.5 to 1.5):(20 to 100):(20 to 100):(1 to 20).

6. The method for preparing the nitrogen-containing conjugated lignin composite material according to claim 1, characterized in that: The mass ratio of the nitrogen-containing conjugated lignin polymer to sulfur powder is 1:(1-10).

7. A nitrogen-containing conjugated lignin composite material prepared by the method of claim 1, characterized in that: The material has a lignin conjugated polymer network structure, and the nitrogen-containing functional groups in the polymer can serve as active sites to inhibit "polysulfide shuttle".

8. Use of the nitrogen-containing conjugated lignin composite material prepared by the method of claim 1 in a lithium-sulfur battery.

9. The use according to claim 8, characterized in that The obtained nitrogen-containing conjugated lignin composite material is ground and mixed with SuperP and polyvinylidene fluoride PVDF to obtain a positive electrode slurry; the obtained positive electrode slurry is then spread on the surface of aluminum foil and vacuum dried to obtain a nitrogen-containing conjugated lignin lithium-sulfur battery positive electrode; wherein, The mass ratio of the nitrogen-containing conjugated lignin composite material to SuperP and polyvinylidene fluoride (PVDF) is (6-9.5):(0.5-2):(0.5-2).

10. The use according to claim 9, characterized in that The nitrogen-containing conjugated lignin positive electrode slurry is scraped with a scraper with a height of 6 to 20 μm; the vacuum drying temperature is 50°C to 80°C, and the vacuum drying time is 6 to 24 hours.

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